§ 156.126. Stormwater Management; General Requirements.

(A)

Protection of the water resources in the City is critical to the public health, safety and welfare. Innovative approaches to storm water management shall be encouraged and the concurrent control of erosion, sedimentation and flooding shall be mandatory.

(B)

The following general criteria shall be applicable to all proposed storm water management plans, for both public and private improvements, as approved by the City Engineer:

(1)

When possible, the nonstructural approach shall be used to meet both surface water quantity and quality requirements.

(2)

The drainage system for each phase of a development shall be capable of standing on its own if subsequent areas planned for development are not developed.

(3)

The drainage system for each development shall be sized to accommodate existing upstream runoff.

(4)

The general criteria, as herein and subsequently outlined, shall, in no way, be construed as prohibiting new and innovative techniques. However, all such new and innovative techniques shall be subject to the approval of the City Engineer at a pre-application conference stage prior to their use in the design of any development.

(5)

The storage and controlled release or retention on site and infiltration into the ground of excess storm water runoff from any commercial, industrial and residential developments will be required so that runoff therefrom will not be substantially greater than it was prior to such development.

(6)

The procedure for disposing of excess storm water runoff shall be dependent on the Hydrologic Soil Classifications of the soils within the proposed development boundaries, which Hydrologic Soil Classification shall be as are used and defined by the Soil Conservation Service of the U.S. Department of Agriculture (A, B, C or D and A/D, B/D and C/D) in the publication, St. Lucie County, Florida - Soils, and in other publications of the Soil Conservation Services. However, the location and designation of the various Soil Types as depicted therein shall be fully substantiated by Soils analysis, if required by the City Engineer.

(7)

All major storm water conveyance systems shall be of the shallow, flat, slow-velocity, open channel, "floodway" type which shall be designed to delay or retard runoff from any development. The side slopes of such conveyance systems shall be kept as flat as possible (maximum of three (3) horizontal to one (1) vertical). The construction of deep artificial canals, ditches, channels and the like, or the construction of major storm sewer systems, which will rapidly convey runoff to any receiving waters or substantially reduce the level of the ground water table, is extremely prohibited. Substantial reduction of the ground water table shall be minimized and determined on a site-specific basis.

(8)

All retention areas, excluding wetlands, shall be landscaped with one (1) tree every fifty (50) linear feet. Trees shall be planted at the slope of the retention area to create a vegetative buffer. Tree location, size and species selection shall be approved by the Site Plan Review Committee.

(9)

Should the proposed development area contain an existing natural watercourse drainageway, channel and the like, such natural watercourse and the vegetation inherent therewith shall be maintained and the proposed development designed so as to preserve same. However, the use of such natural watercourse to carry off runoff from any development shall be permitted if provision for control of sediment in the excess runoff is made prior to entrance of the runoff to the natural watercourse. This does not preclude the use of wetlands for storage and treatment of stormwater runoff as long as the designed drainage system does not measurably degrade the affected area.

(10)

Computations for street drainage and culverts shall be based upon one-day rainfall, ten-year return period.

(11)

Street inlets shall be spaced in such a manner as to accept one hundred (100) percent of the design runoff and shall be separated by no more than four hundred (400) feet. Typically, the maximum allowable gutter run will be one thousand two hundred (1,200) feet on streets with standard curbs, and six hundred (600) feet on streets where Miami-type curbs are approved for special application by the City Engineer. However, the actual required spacing will depend on the characteristics of such particular site and the flow quantities to be handled.

(12)

Runoff from driveways, roofs or other impervious areas shall be diverted so as to flow over grassed areas prior to flowing into any drainage system whenever possible.

(13)

All detention and retention basins shall be readily accessible from streets of public right-of-way and shall be situated so that maintenance can be easily performed.

(14)

The side slopes of all detention basins shall be kept as flat as possible (maximum of four (4) horizontal to one (1) vertical), providing soil conditions are suitable to sustain plant growth and control erosion.

(15)

No site alteration shall adversely affect the existing surface water flow pattern.

(16)

No site alteration shall cause siltation of wetlands, pollution of downstream wetlands or reduce the natural retention or filtering capabilities of wetlands.

(17)

No site alteration shall allow water to become a health hazard.

(18)

All site alteration activities shall provide for such water retention and settling structures and flow-attenuation devices as may be necessary to insure that the foregoing standards and requirements are met.

(19)

Where necessary, easements for drainage facilities, as approved by the City Engineer, shall be provided as follows:

(a)

Any off-site easements, which are needed to make the drainage system function, shall be included in the proposal for development and made a criteria for preliminary and final plat approval.

(b)

Easements for all facilities must be shown on construction drawings and approved by the City Engineer. The easements and rights-of-way must be executed, accepted by the City Council, and recorded in the public records prior to issuance of a building permit.

(c)

Alternative storm water management and conservation control strategies shall meet the following performance standards and requirements:

1.

The outflow hydrograph, as computed for the developed or undeveloped area, shall approximate (within ten (10%) percent in terms of peak flow) the hydrograph of conditions existing before development or redevelopment for the ten-year Frequency Three-Hour Design Rainfalls prior to its entrance into any watercourse outside the developed or redeveloped area. In addition, the existing shape, timing and effect of the outflow hydrographs on downstream flow shall be maintained as much as possible. Runoff rates and volumes resulting from the development, in excess of existing amounts, shall be accommodated in an approved manner on-site.

2.

The capacity of any outlet watercourse beyond the area of the developed or redeveloped area shall be adequate to convey the peak discharge from a 25-Year Frequency Six-Hour Design Rainfall. If the downstream facilities are inadequate to convey the peak discharge for the design rainfalls above, the proposed development must accommodate that portion of runoff above the downstream systems actual capacity.

3.

Permitted rates and volumes of storm water runoff, whether discharged into natural or artificial watercourses, shall meet existing water quality standards at the first downstream receiving water body for which such standards have been established.

4.

Disposition of Storm Water in Impervious Areas. The areas where the soils have been classified under the SCS Hydrologic Soils Classification System as types C and D (impervious) or A/D, B/D and C/D (high ground water table areas), the overall storm water management system shall be that of providing detention basins to attenuate peak from the contributory drainage area and to settle solids washed off or eroded therefrom. Specific guidelines area as follows:

a.

Size. Detention basins shall have sufficient storage so as to contain the increased runoff after development based upon the 25-year return period, three-day rainfall minus the allowable discharge with a minimum freeboard of two (2) feet in the basis. Minor flooding of the area and streets in the vicinity of the detention basin during the 100-Year Frequency—24-Hour Design Rainfall will be permitted. The capacity of the outlet structure shall be considered in all cases.

b.

Outlet Structure. Outlet structures shall be as simple as possible (pipe culverts, concrete weirs and the like). Required detention storage in the basin, as defined above, shall be above the invert of the outlet structure. When combined with the detention storage required for the 25-Year Frequency - Six-Hour Design Rainfall the outlet structure shall be of sufficient size to permit seventy-five (75%) percent of the total volume of water temporarily stored in the basin to drain out within twenty-four (24) hours from the end of the theoretical storm. Consideration must be given not to adversely restrict predevelopment flow off-site.

c.

Configuration. The shape of all detention basins shall be such that no "short-circuiting" of flow occurs and that maximum disposition of suspended solids is achieved in the basin. Dredging of the settled sediments will be carried out when necessary so that the capacity of the basin is not reduced below that for which it is designed.

5.

Disposition of Storm Water in Pervious Areas. In areas where soils have been classified under the SCS Hydrologic Soil Classification System as Types A or B (pervious), the overall storm water management strategy shall be that of on-site retention and infiltration into the ground. Specific guidelines are as follows:

a.

On-Lot Infiltration. The parcel shall be developed to maximize the amount of natural rainfall which is infiltrated into the soil and to minimize direct overland runoff into adjoining streets and watercourses. Storm water runoff from roofs and other impervious surfaces could be diverted into swales or terraces on the lot when possible.

b.

Retention Basins. Retention basins shall have sufficient storage so as to contain the increased runoff after development, based upon the 25-year return period, three-day rainfall minus the allowable discharge, with a minimum freeboard of two (2) feet considering that infiltration out of the basin through the side and bottom is occurring during these storms. Minor flooding of the area and streets in the vicinity of the retention basin during the theoretical 100-Year Frequency - 24 Hour Design Rainfall will be permitted.

c.

Outlet structure. No actual outlet from a retention basin will be required where such basins are located in soils classified as Type A. However, possible overflow from the basin shall be anticipated and a path for such overflow provided and structures in the development so situated that no flood damage thereto will occur. Retention basins is classified as Type B shall have nominal positive outlet to other basins and/or watercourses. However, the nominal positive outlet shall be assumed to have no capacity when determining retention basin size.

(Ord. 94-7, passed 3-14-94)

Cross reference

Penalty, see section 156.999.